Supplemental Electrode - One or Two Required?

Metal underground water pipes have been used as zero reference points or natural electrodes for electrical systems supplying buildings and other structures for many years without problems. When a metal water pipe deteriorated with age or from corrosion, a new metal water pipe was installed in its place. With the introduction of nonmetallic water piping systems in new construction and the replacement of deteriorated metal water pipes with nonmetallic ones for existing buildings, underground metal water pipes should not be relied upon as the only electrode for the electrical installation. With changes in usage and installation materials for grounding electrodes, procedures must also change to ensure that proper ground reference for the electrical system is maintained. For a metal underground water piping system to function as a proper grounding electrode, the metal water pipe must be in direct contact with the earth for at least 10 feet. This is a critical part of the grounding electrode, since contact with the earth and moisture surrounding the metal water pipe is providing the grounding reference of the electrode. Metal water pipe to earth contact of less than 10 feet may not provide the zero reference to ground desired for the electrical service. The metal piping system must be, or be made, electrically continuous by bonding around any insulating joints or nonmetallic pieces of pipe that may interrupt the continuity of the metal pipe. For example, a dielectric union may have been installed in the metal water pipe to convert from galvanized or black steel pipe to copper. The dielectric union provides an insulator between the two different metal piping systems to counter the effects of galvanic corrosion or cathodic action (one metal acting as a sacrificial element to the other metal). This would otherwise result in deterioration of one or the other pipes. The movement of water, acting as an electrolyte, past the two dissimilar metals causes a small DC voltage and thus current flows at the junction or coupling of the two metals. The small DC current that results is similar to a battery (one of the metals becomes an anode and the other a cathode) with eventual breakdown of one of the metals due to the resultant chemical reaction. Since the dielectric union insulates one length of pipe from the other at the connection point, a bonding conductor must be installed around the insulator. A small amount of current may still occur with the bonding jumper, but the cathodic effect is minimized. Because the metal underground water pipe may be replaced eventually with a nonmetallic piping system, a backup or supplementary electrode has been required for the electrical system by the National Electrical Code (NEC) for quite some time. This supplementary electrode can be an electrode of any type permitted by Section 250-50, such as building steel, a concrete encased electrode, or a ground ring. Because these electrodes must already be connected to the electrical system if they are available at the premises, the electrodes most likely to be used as supplementary electrodes would be man-made electrodes. One of these man-made electrodes could be metal underground tanks or other metal piping systems described in Section 250-52(b), although galvanic corrosion can occur here also, creating additional problems. Metal underground gas pipes must never be used as an electrode since any galvanic corrosion of the pipe could have disastrous results. The two most common electrodes used to supplement the metal water pipe electrode are rod electrodes or plate electrodes as described in Sections 250-52(c) and (d), respectively. A rod electrode must not be less than 8 feet in length and can consist of iron, steel, stainless steel, nonferrous metals (metals that do not contain iron), or a combination of these materials. For example, a steel rod may be manufactured with a nonferrous outer covering of copper or a copper alloy. If a steel or iron rod is used, the rod must be at least 5/8 inches in diameter and, of course, at least 8 feet in length. Stainless steel rods, nonferrous rods, or their equivalent must be listed and must not be less than 1/2 inch in diameter. These rod electrodes must be driven into the ground such that at least 8 feet of rod is in direct contact with the earth. A plate electrode, covered in Section 250-52(d), is not used as a supplemental electrode as often as a rod electrode. If used, however, the plate electrode must expose not less than 2 square feet of plate surface to exterior soil. Also, the plate electrode must be installed at exterior soil unlike rod electrodes that can be installed inside of a building by drilling a hole through the concrete floor and driving the rod into the earth. Plate electrodes, like rod electrodes, can be of iron, steel, and nonferrous materials. The 1999 NEC has added a 21/2-foot depth requirement for plate electrodes to ensure the plate is exposed on both sides to the earth to establish a reference to ground that is as close as possible to zero resistance.These rod, pipe, or plate electrodes may be installed in dry, sandy soil or in other soil conditions that do not providea low resistance to earth. Section 250-56 requires that an additional electrode must be installed if the first electrode does not have a resistance to ground of 25 ohms or less. If these electrodes were being installed as the only electrodes for an electrical service and the resistance to ground of the rod or plate exceeded 25 ohms, an additional electrode would be required. Taking the next logical step, if these rods were installed as supplemental rods for the metal underground water pipe, assuming a replacement using nonmetallic water pipe in the future, the rods would then become the primary electrodes. In the 1999 NEC, Section 250-50(a)(1) now requires compliance with Section 250-56 for even supplementary electrodes. In other words, rod, pipe, or plate electrodes used as supplementary electrodes must have a reference to ground of not greater than 25 ohms or an additional electrode must be installed. With all of the changes and the rewritten text in Article 250 for the 1999 NEC, it is fairly easy to overlook a change such as this. It may be a good idea to study some of the sections in Article 250 that you commonly use, compare the text to the 1996 text, and ensure you are not missing something that may affect your electrical installation. ODE is a staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or by e-mail at

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